Topic 8: Energy, power and climate change

1. 8.4 Non-fossil Fuel Production Topic 8: Energy, power and climate change

2. Nuclear Power Solar Power Hydroelectric Power Wind Power Wave Power Non-Fossil fuel production

3. Chain reactions

4. Chain reactions can only take place if more neutrons are released than were used during the nuclear reaction. Isotopes that produce an excess of neutrons in their fission support a chain reaction. This type of isotope is said to be fissionable, Only two main fissionable isotopes are used during nuclear reactions — uranium-235 and plutonium-239. The minimum amount of fissionable material needed to ensure that a chain reaction occurs is called the critical mass.

5. Controlled fission To maintain a sustained controlled nuclear reaction, there must be at least one neutron from each fission being absorbed by another fissionable nucleus. The reaction can be controlled by using control rods of material which absorbs neutrons. Control rods are commonly made of a strongly neutron-absorbent material such as boron or cadmium.

6. Uncontrolled fission A fission reaction whereby the reaction is allowed to proceed without any moderation or control rods is called an uncontrolled fission reaction . If there are too many neutrons, the chain reaction would proceed at tremendous pace and result in an explosion. An example would be in an atomic bomb where the reactions are uncontrolled. In a nuclear reactor, if the fission process is not well controlled, the large amounts of energy would cause the fuel to melt and set fire to the reactor in what is called a meltdown.

7. Fuel enrichment Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in nuclear reactors is from the 'fission' or splitting of the U-235 atoms, a process which releases energy in the form of heat. U-235 is the main fissile isotope of uranium. Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium).

8. Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel. Most present day reactors (Light Water Reactors or LWRs) use enriched uranium where the proportion of the U-235 isotope has been increased from 0.7% to about 3% or up to 5%. For comparison, uranium used for nuclear weapons would have to be enriched in plants specially designed to produce at least 90% U-235.

9. Energy transformations in a nuclear power station

10. Sankey diagrams for energy efficiency in a nuclear power plant

11. The nuclear fuel cycle

12. Main stages in the nuclear fuel cycle Uranium recovery to extract (or mine) uranium ore, and concentrate (or mill) the ore to produce "yellowcake" Conversion of yellowcake into uranium hexafluoride (UF6) Enrichment to increase the concentration of uranium-235 (U235) in UF6 Fuel fabrication to convert enriched UF6 into fuel (pellets) for nuclear reactors Use of the fuel in reactors (nuclear power, research, or naval propulsion) Interim storage of spent nuclear fuel Reprocessing of high-level waste (currently not done in the U.S.) [1] Final disposition (disposal) of high-level waste

13. Role of control rods The control rods, an important part of the reactor, regulate or control the speed of the nuclear chain reaction, by sliding up and down between the fuel rods or fuel assemblies in the reactor core.  • The control rods contain material such as cadmium and boron.  Because of their atomic structure cadmium and boron absorb neutrons, but do not fission or split.

14. The temperature in the reactor core is carefully monitored and controlled.  When the core temperature goes down, the control rods are slowly lifted out of the core, and fewer neutrons are absorbed.  Therefore, more neutrons are available to cause fission.  This releases more energy and heat.  When the temperature in the core rises, the rods are slowly lowered and the energy output decreases because fewer neutrons are available for the chain reaction -- the control rods absorb neutrons that could otherwise hit uranium atoms and cause them to split.  To maintain a controlled nuclear chain reaction, the control rods are manipulated in such a way that each fission will result in just one neutron, since the other neutrons are absorbed by the control rods.

15. Role of moderator In addition to the need to capture neturons, the neutrons often have too much kinetic energy. These fast neutrons are slowed through the use of a moderator such as heavy water and ordinary water. Some reactors use graphite as a moderator, but this design has several problems. Once the fast neutrons have been slowed, they are more likely to produce further nuclear fissions or be absorbed by the control rod. Java applet nuclear reaction http://library.thinkquest.org/17940/texts/java/Reaction.html

16. A nuclear reactor Inside the "core" where the nuclear reactions take place are the fuel rods and assemblies, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and part of the cooling system.

17. The job of the coolant is to absorb the heat from the reaction. The most common coolant used in nuclear power plants today is water. In actuality, in many reactor designs the coolant and the moderator are one and the same. The coolant water is heated by the nuclear reactions going on inside the core. However, this heated water does not boil because it is kept at an extremely intense pressure, thus raising its boiling point above the normal 100° Celsius.

18. Heat exchanger A heat exchanger is a device built for efficient heat transfer from one medium to another The heated water rises up and passes through another part of the reactor, the heat exchanger. The moderator/coolant water is radioactive, so it can not leave the inner reactor containment. Its heat must be transferred to non-radioactive water, which can then be sent out of the reactor shielding.

19. This is done through the heat exchanger, which works by moving the radioactive water through a series of pipes that are wrapped around other pipes. The metallic pipes conduct the heat from the moderator to the normal water. Then, the normal water (now in steam form and intensely hot) moves to the turbine, where electricity is produced.

20. We are not able to convert all the internal energy of the system into useful work but we can extract some useful work through heat engines. The temperature of the reactor is typically limited to 570K. Higher temperature tend to damage the fuel rods. Typically the temperature of the water returning to the heat exchanger is 310K The efficiency of the nuclear plant is about 46% With further energy used to drive pumps and pollution control devices, the efficiency is usually reduced to 34%

21. Plutonium-239 U-238 is not fissile but it is useful because it can be used to produced Pu-239, a fissionable isotope. First, U-238 becomes U-239 by neutron capture: Then U-239 goes through beta decay to become Neptunium

22. Then Neptunium beta decays into Plutonium And Pu-239 is fissionable and large amounts of energy is released

23. Plutonium-239 as a nuclear fuel U-238 is 140 times more abundant than U-235. The neutrons given off in a U-235 reaction can be used to “breed” more fuel if the non-fissionable U-238 is placed in a “blanket” around the control rods containing U-235. On average, 2.4 neutrons are produced in a U-235 reaction with 1 neutron required for the next fission and 1.4 left for neutron capture by U-238.

24. Suppose there were 100 fissions of U-235 and 240 neutrons are produced. 100 neutrons will be needed to start the next fission of U-235 and 140 neutrons will be available for neutron capture. Suppose that some neutrons are lost and there are 110 neutrons available for capture by non-fissionable U-238. This means that there will be 110 fissions of Pu-239. Therefore 100 U-235 will produce 110 fissions of Pu-239, which is a 10% increase in fuel.

25. Safety and risks of nuclear power Problems associated with mining of Uranium Problems with disposal Risk of thermal meltdown Risk of nuclear programs as means of nuclear weapon production

26. Biggest risk for mining of uranium is the exposure of miners to radon-222 gas and other highly radioactive products, as well as water containing radioactive and toxic materials In 1950s, a significant number of american miners developed small cell lung cancer due to radon which was the cancer causing agent.

27. The are concerns over the disposal of waste : - Low-level (radioactive cooling water, lab equipment and protective clothing) - Intermediate level (coolant) - High level (fuel rods) The products of fission called “ash” include isotopes of strongtium, caesium and krypton which are highly radioactive with half lives of 30 years or less.

28. The biggest concern is Pu-239 which has a half-life of approx 24,000 years. • It is also used in nuclear warheads • Presently the disposal methods include deep storage underground. • If these methods fail, there would be catastrophic consequences • Radioactive waste would find its way into the food chain and underground water would become contaminated.

29. Provided that reactors are built to standard and maintained properly, no obvious pollutants escape into the atmosphere that would contribute to the “greenhouse” effect. However, even with expensive cooling towers and cooling ponds, thermal pollution from the heat produced by the exchanger process could contribute to global warming. The disadvantage of possible nuclear power plant containment failure is always present. Nuclear terrorism is a threat.

30. Nuclear power using nuclear fusion The most probable way is to fuse deuterium and tritium. Deuterium atoms can be extracted from seawater and tritium can be bred from lithium.

31. Nuclear power using nuclear fusion? The basic problems in attaining useful nuclear fusion conditions are to heat the gas to these very high temperatures and to confine a sufficient quantity of the reacting nuclei for a long enough time to permit the release of more energy than is needed to heat and confine the gas. the capture of this energy and its conversion to electricity.

32. Nuclear power using nuclear fusion? Nuclear fusion was first achieved on earth in the early 1930s by bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with high-energy deuterons in a cyclotron (Particle accelerator). To accelerate the deuteron beam a great deal of energy is required, most of which appeared as heat in the target. As a result, no net useful energy was produced. In the 1950s the first large-scale but uncontrolled release of fusion energy was demonstrated in the tests of thermonuclear weapons by the United States, the USSR, the United Kingdom, and France. This was such a brief and uncontrolled release that it could not be used for the production of electric power

33. The problem with fusion is the sheer difficulty of achieving the act. Why the very high temperatures? Atoms have a very strong repulsive force and it takes high temperatures and enormous amounts of energy to bring them close enough together to fuse. And this must be maintained for long periods to produce electricity. We have been researching fusion for over four decades and spent many millions of dollars, pounds and euros. It is possible that more money and time could produce successful fusion in another decade or so, but it may never be achievable. It might be wiser to spend that time and money on something which we know will succeed such as renewables.

34. At temperatures of 100,000° C, all the hydrogen atoms are fully ionized. The gas consists of an electrically neutral assemblage of positively charged nuclei and negatively charged free electrons. This state of matter is called a plasma. A plasma hot enough for fusion cannot be contained by ordinary materials. The plasma would cool very rapidly, and the vessel walls would be destroyed by the extreme heat. However, since the plasma consists of charged nuclei and electrons, which move in tight spirals around the lines of force of strong magnetic fields, the plasma can be contained in a properly shaped magnetic field region without reacting with material walls. Why containment?

35. Why is high temp maintained? Because fusion is not a chain reaction, these temperature and density conditions have to be maintained for future fusion to occur.

36. If fusion energy does become practical, it offers the following advantages: a limitless source of fuel, deuterium from the ocean; no possibility of a reactor accident, as the amount of fuel in the system is very small; and waste products much less radioactive and simpler to handle than those from fission systems.

37. Photovoltaic cells Photovoltaic devices make use of the photoelectric effect. Solar photovoltaic modules use solar cells to convert light from the sun into electricity.

38. Solar heating panels Solar thermal panels contain liquid that circulates through special panels and is heated by sunlight, this then passes through a coil in the water tank which in turn heats the water stored in the tank

39. What are the factors that would affect the amount of solar radiation that a place gets?

40. The main factors are: Geographic location Time of day (altitude of the sun from the sky) Season Local landscape Local weather The distance of earth from the sun

41. Because the Earth is round, the sun strikes the surface at different angles ranging from 0º (just above the horizon) to 90º (directly overhead). When the sun's rays are vertical, the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year

42. The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth's surface receives a little more solar energy. The Earth is nearer the sun when it's summer in the southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.

43. The 23.5º tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other six months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.

44. Countries like the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more solar energy in June than they do in December

45. The rotation of the Earth is responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon when the sun is at its highest point. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon

46. 3 main schemes Water storage in lakes Tidal water storage Pump storage

47. Water storage in lakes

48. Water storage in lakes The Three Gorges Dam on the Yangtze River will be the largest hydroelectric dam in the world when it is complete in 2009. It will generate 18200MW The dam is more than 2 km wide and has a height of 185m. Its reservoir will stretch over 600km upstream and force the displacement of more than 1.3million people.

49. Tidal water storage Have been built in Russia and France and in developmental stage in other countries Source of energy is the kinetic energy of the earth’s rotation. Coastal estuaries that have a large vertical range in tides are potential sites for tidal power stations The station in France has a tidal range of 8.4m and generates 10MW of electrical energy for each of the 24 turbines.

50. Tidal water storage A dam is built to catch the high tide. A sluice gate is opened to let the high tide water in The water is released at low tide, and the gravitational potential energy is used to drive turbines which produce electrical energy